A minimum pressure valve which can be applied to the outlet of a liquid separator is already known, whereby this minimum pressure valve is provided with a housing with an inlet and an outlet which are connected via an inner duct system and whereby, in the above-mentioned duct system, a movable valve body is provided which is pressed against a seating round the aforesaid inlet in a first, closed position by means of a spring and thereby closes off the inlet; and whereby the valve body is situated at a distance from the above-mentioned seating in a second, open position, such that the inlet, via the duct system, is connected to the outlet.
The presence of the above-mentioned spring makes sure that the minimum pressure valve only opens when a certain minimum pressure has been reached at the inlet of this minimum pressure valve, which inlet may be connected for example to the outlet of a liquid separator.
The reason therefore is that the gas speeds through the liquid separator must be restricted in order to avoid damage to the filter paper and loss of the separation efficiency.
By realizing the above-mentioned spring in the shape of a spring having a specific spring constant and thanks to an appropriate installation of the spring, the opening pressure of the minimum pressure valve can be selected as a function of the application.
Via the inlet, a force is exerted on the valve body by the gas pressure at the inlet, whereas a spring force is exerted in the opposite sense on the other side of the valve body by the above-mentioned spring, which spring force makes sure that the minimum pressure valve remains closed until the vessel pressure has reached a certain value.
A disadvantage of the existing minimum pressure valves is that the spring must be dimensioned such that it is capable of compensating the entire pressure force of the gas at the gas inlet.
Since these pressure values may be relatively large, for example between 4 and 9 bar (±4.105 to 9.105 Pa) for an oil-injected compressor and even up to 20 bar (±20.105 Pa) for water-injected compressors, acting on relatively large surfaces, strong springs must be selected.
Moreover, the overall height of the spring must be restricted so as to limit the total height of the minimum pressure valve, and to restrict the material costs and weight.
One must also take into account that the opening range of the minimum pressure valve is small and that, consequently, little extra gas pressure is required to push the valve body entirely open, since the pressure drop over the minimum pressure valve is preferably as small as possible for the entire service range of the compressor (i.e. from minimum operating pressure to maximum operating pressure). In other words, the valve body must reach the entirely open position as fast as possible as soon as the minimum pressure valve begins to open, in order to cause a pressure drop that is as small as possible over the entire pressure range of the compressor.
However, the two above-described conditions are contradictory, since a small overall height for a large required spring force automatically leads to a spring having a large spring constant, which always results in a large difference in spring force between the initial opening and the full opening, such that the minimum pressure valve will only be entirely open at a pressure which is considerably higher than the pressure whereby the valve only just begins to open, such that the pressure drop over the valve is large in the lower part of the service range.